Rust: What is the difference between len and capacity?

len — number of elements. capacity — allocated space.

len:

• Actual elements.
• Always ≤ capacity.
• For count.

capacity:

• Allocated space.
• May be > len.
• For pre-allocation.

1let mut v = Vec::with_capacity(10);
2v.push(1);
3v.push(2);
4println!("len: {}, cap: {}", v.len(), v.capacity());
5// len: 2, cap: 10

Key: len for elements, capacity for allocated space.